1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to systems and techniques for locating the position of a wireless communication device in a code division multiple access system.
2. Description of the Related Art
Deployment of location technologies in wireless networks is being driven by regulatory forces and carriers"" desires to enhance revenues by differentiating the services offered by one carrier from the services offered by others. In addition, in June 1996, the Federal Communications Commission (FCC) mandated support for enhanced emergency 911 (E-911) service. Phase I of the Order requires that sector and cell information be set back to the PSAP (Public Safety Answering Point) agency. Phase II of the Order requires that the location of the cellular transceiver be sent back to the PSAP. To comply with the FCC mandate, 77,000 total sites are to be equipped with automatic location technologies by the year 2005.
Many techniques are being considered to provide automatic location capability. One technique being considered involves measuring the time difference of arrival of signals from a number of cell sites. These signals are triangulated to extract location information. Unfortunately, this technique requires a high concentration of cell sites and/or an increase in the transmission power of the sites to be effective. This is due to the fact that in a typical CDMA system, each telephone transmits with only enough signal power to reach the closest cell site. As triangulation requires communication with at least three sites, the concentration of cell sites would have to be increased or the signal power of each wireless communication device would have to be increased.
In any event, each alternative has significant drawbacks. An increase in the number of cell sites would be too costly. Increases in signal power would add to the weight and cost of each wireless communication device and increase the likelihood of interference between wireless users. In addition, the network triangulation approach does not appear to meet the FCC mandate requirements.
Another approach being considered involves the addition of GPS (Global Positioning System) functionality to the cellular telephone. Although this approach would add significant cost and weight to the wireless communication device, require a line-of-sight to four satellites, and would be somewhat slow, nevertheless, it is the most accurate approach to support location services.
To speed the process, a third approach sends aiding information to the wireless communication device indicating where the wireless communication device should look in frequency for GPS carriers. Most GPS receivers use what is known as a GPS satellite almanac to minimize a search performed by the receiver in the frequency domain for a signal from a visible satellite. The almanac is a 15,000 bit block of coarse ephemeris and time model data for the entire constellation. The information in the almanac regarding the position of the satellite and the current time of day is approximate only. Without an almanac, the GPS receiver must conduct the widest possible frequency search to acquire a satellite signal. Additional processing is required to attain additional information that will aid in acquiring other satellites.
The signal acquisition process can take several minutes due to the large number of frequency bins that need to be searched. Each frequency bin has a center frequency and predefined width. The availability of the almanac reduces the uncertainty in satellite Doppler and therefore the number of bins that must be searched.
The satellite almanac can be extracted from the GPS navigation message or sent on the down (forward) link as a data or signaling message to the receiver. Upon receipt of this information, the receiver performs GPS signal processing to determine its location. While this approach may be somewhat faster, it suffers from the requirement of a line-of-sight to at least four satellites. This may be problematic in urban environments.
Hence, a need remains in the art for a fast, accurate and inexpensive system or technique for locating a cellular.
The need in the art is addressed by the system and method presently disclosed for determining the position of a wireless transceiver. In the most general sense, the inventive method is a hybrid approach for determining position using ranging information from a terrestrial system, timing information from a wireless communication device, and ranging information from GPS satellites. This information is combined to allow the position of a wireless communication device to rapidly and reliably determined. The disclosed method includes the steps of receiving at a wireless communication device, a first signal transmitted from a first GPS satellite, a second signal transmitted from a second GPS satellite, and a third signal form a third satellite. The wireless communication device is adapted to receive these GPS signals and transmit a fourth signal to the base station in response thereto. The base station receives the fourth signal, corrects for the clock bias imposed on the fourth signal by the round trip delay between the base station and the wireless communication device and uses the unbiased fourth signal to calculate the position of the wireless communication device.
In a specific implementation, the base station sends aiding information to the wireless communication device. The aiding information is used by the wireless communication device to quickly acquire the signals transmitted by the first, second and third satellites. The aiding signals are derived from information collected at the base station transceiver subsystem (BTS) serving the wireless communication device, Base Station Controller (BSC), or some other entity and includes: (1) satellite identification information; (2) Doppler shift or related information; (3) values indicating the distance between the base station and each satellite; and (4) a search window size associated with each satellite, the search window size being calculated based on the round trip delay between the wireless communication device and the base station and the elevation angle of each satellite.
Upon acquisition by the wireless communication device of the signals transmitted by the first, second and third satellites, the wireless communication device calculates the range pm1, between the wireless communication device and the first satellite, range pm2 between the wireless communication device and the second satellite, and range pm3 between the wireless communication device and the third satellite. This range information is transmitted back to the base station along with information as to the time at which the measurement was made. In a CDMA implementation, the time it takes the signal to propagate between the base station antenna and the wireless communication device antenna is half the round trip delay and is known by the base station. A measure of the round trip delay between the wireless communication device and the base station indicates the distance between the wireless communication device and the base station. In addition, this delay provides a means for correcting the wireless communication device absolute time.
A device external to the wireless communication device, such as the base station controller or some other entity associated with the cellular infrastructure, utilizes information known to the serving base station to calculate the position of the wireless communication device. Such information may include the position of the first, second, and third satellites relative to the wireless communication device and the distance between the wireless communication device and the base station. Determining the position of the wireless communication device is achieved by finding: (1) an intersection of a first sphere of radius cp1 around a first satellite, (2) a second sphere of radii cp2 around the second satellite, (3) a third sphere of radii cp3 around the third satellite, and (4) a fourth sphere of radius cpb around the base station. xe2x80x9ccxe2x80x9d is the speed of light, xe2x80x9cp1xe2x80x9d is the pseudo-range associated with the first satellite and the wireless communication device, xe2x80x9cp2 xe2x80x9d is the pseudo-range associated with the second satellite and the wireless communication device, xe2x80x9cp3 xe2x80x9d is the pseudo-range associated with the third satellite and the wireless communication device, and xe2x80x9ccpbxe2x80x9d is the pseudo-range associated with the base station and the wireless communication device.
Note that if a line-of-sight (no multipath) exists between the wireless communication device and the base station, then the proposed approach requires measurements from only two satellites and one base station. In the case of a communication system that is synchronized to GPS time, such as a CDMA communication system, the pseudo-range measurement taken from the signals transmitted by the base station will be used both to remove the bias from the satellite pseudo-range measurements and as an additional ranging measurement. Additional information from another base station, if available, can be used to further reduce the number of satellites required to determine the position of the wireless communication device. Also in situations, where only two-dimensional locations are needed, only one satellite and one base station are needed.
One key advantage of this approach over other known GPS approaches is the speed with which the wireless communication device can determine the pseudo-range. Since the serving base station transceiver, base station controller, or other entity coupled to the base station has its own GPS receiver, and also knows the pseudo-ranges of all satellites being tracked with respect to the serving base station location, it is possible to determine a search window center and search window size for each satellite being tracked. The information is sent to the wireless communication device to increase the speed of the search process.
That is, a clock onboard each GPS satellite controls the timing of the broadcast of the ranging signal by the satellite. Each such clock is synchronized to GPS system time. The base station also contains a clock that is synchronized to GPS system time. The wireless communication device synchronizes its clock to GPS time with a delay corresponding to the one-way delay between the base station and the wireless communication device. Timing information is embedded within the satellite ranging signal that enables the wireless communication device to calculate when the signal was transmitted from a specific satellite. By recording the time when the signal was received, the distance (range) from the satellite to the wireless communication device can be computed. As a result, the locus of the location of the wireless communication device is a sphere with center at the satellite location and radius equal to the calculated range. If a measurement is simultaneously made using the ranging of two other satellites, the wireless communication device would be somewhere on the surface of three spheres. The three spheres intersects in two points, however, only one of the points is the correct wireless user position. The candidate locations are mirror images of one another with respect to the plane containing the three satellites.
In one embodiment of the disclosed method and apparatus, the GPS satellites for locating the position of the wireless communication device at a given point in time are identified by the base station. This information is forwarded to the wireless communication device to facilitate the search operation performed by the wireless communication device.
In addition to the above, when the wireless communication device is a Code Division Multiple Access (CDMA) receiver, the presently disclosed method and apparatus takes advantage of the fact that CDMA is a synchronous system. Being synchronous, the time of arrival of a reference pilot at the wireless communication device can be used as a time reference. Accordingly, the wireless communication device can measure the time difference of arrival between the reference pilot, GPS signals, and other pilot signals. Accordingly, the problem of determining the location of the wireless communication device becomes a time difference of arrival (TDOA) problem, resulting in a further reduction in the number of satellites required to determine the location of the wireless communication device.
In one embodiment, the wireless communication device can have several modes of operation:
(1) Hybrid mode using information from both the wireless system infrastructure and the GPS satellites;
(2) Stand-alone (standard or conventional) GPS mode;
(3) Aided stand-alone GPS mode;
(4) Inverted differential GPS mode; and
(5) Aided and inverted differential GPS mode.